EP0394932B1 - Photothermal inspection method, arrangement for its working out, and utilisation of the method - Google Patents

Abstract

Situated inside a portable measuring head (A) is the laser light source (FL), preferably a directly modulated, diode-pumped neodymium/YAG laser. Its laser beam (f1) is directed without optical waveguides using a scanner device (5) and a dual optical system (6) onto the scanning zone of the material sample (B) to be inspected. The irradiated measuring track is, for example, meander-shaped with beam deflection in the x and y direction. The dual optical system (6) transmits the IR light signals (- DELTA f3) emitted by the sample (B). These are focussed on an IR detector (8) via the scanner device (5) and a coupling mirror (4b) in the form of a dichroic mirror which reflects the laser beams (f1) in one direction and acts as a transparent window in relation to the IR light signals (- DELTA f3) in the other direction, and via an IR optical system (7). The IR detector (8) converts the IR light signals (- DELTA f3) into corresponding electrical signals. The measuring head (A) is connected via a flexible electrical connecting cable to an electronic image generating unit from which the electrical signals for controlling the laser (FL) and guiding the beam originate. <??>The invention also relates to a number of advantageous uses of the method and also to a device for carrying it out. <IMAGE>

Description

The invention relates to a method for testing the properties of materials after the photothermal Effect such as are known from EP-A-105078, US-A-3,803,413 or WO-A-8200891.

ω =

Kreisfrequenz der Modulation des intensitätsmodulierten Laserstrahls

a =

Temperaturleitfähigkeit, wobei für a gilt:

a =

k/ ρ · c, mit

c =

spezifische Wärme

ρ =

Dichte

k =

Wärmeleitfähigkeit des Prüflings.

The basic principle of photothermal examination or measurement methods is based on the irradiation of a test surface with light, in particular with laser light, and evaluation of the heat signals generated thereby in the layers closest to the surface. The fact is used that a body that is warmed in relation to its surroundings always strives to give off this extra amount of heat: the body emits heat in the form of infrared radiation. In principle, the method can also be used if the temperature of the environment is higher than that of the test specimen because the temperature distribution on the test specimen surface is important. By measuring the infrared (IR) light signals emitted by the test object, depth information and information about the material properties of the surface can be obtained, for example it can be determined: changes in the layer thickness of surfaces, but also cracks, inclusions and delaminations, all of which are naturally destructive and non-contact. The invention is based on the fundamentally known method steps of irradiating the surface of the material sample with an intensive light source, in particular a laser, the beam being modulated, ie, in particular periodically interrupted, in a number of photothermal examination methods. The surface of the laser light is partially converted into heat. This heat penetrates into the material sample. A characteristic of the measurement signal formed from emitted IR light signals is how far the heat penetrates. This depends on the one hand on the periodic radiation duration, which is determined by the modulation frequency, and on the other hand on the material properties of thermal conductivity, specific heat and density. The latter three parameters are combined into a physical quantity, the thermal diffusion length μ _S. It directly indicates the depth of penetration of the heat waves. It applies µ S = (2a / ω) , With

ω =

Angular frequency of the modulation of the intensity-modulated laser beam

a =

Thermal conductivity, where for a:

a =

k / ρ · c, with

c =

Specific heat

ρ =

density

k =

Thermal conductivity of the test object.

EP-A1-0 105 078 discloses a device for carrying out the described method for examining the properties of absorbent materials according to the photothermal effect, in which the laser waves from a stationary or quasi-stationary laser to a test head that can be optically coupled to the material sample can be transported using flexible fiber optic cables. In addition, the IR light signals can also be transported from the test head to an infrared detector arranged remotely from the test head via flexible optical waveguide cables. The measuring head itself contains housing-internal beam-guiding means, specifically at the input of the laser beams and at the output of the IR light signals, here a focusing lens each, and also a coupling mirror arranged in the beam path of both lenses, designed as a dichroic mirror ¹⁾ . Furthermore, a crystal rod, in particular made of sapphire, is arranged in the light path common to the laser beams and the IR light signals, and this rod is followed by a focusing lens which focuses the laser radiation on the material sample or receives the IR light signals from it. The optical fibers represent a sensitive element in the material examination; they also attenuate the laser radiation or IR light signals they carry.

The invention has for its object a method for Examining the properties of materials after the photothermal To create effect with which without the line the laser waves via optical fibers from an external stationary or quasi-stationary laser to the measuring head and without one Connection from an external infrared light detector to the measuring head can be managed via fiber optic cables. A another object of the invention is the management of Laser beam from the laser light source on the one hand to the material sample on the other hand, as well as the guidance of the IR light signals from the To design material sample to the infrared light detector so that beam paths are as short as possible and the possibility is opened to couple a second laser light source.

The task is in the procedure mentioned above to investigate the properties of materials after the photothermal Effect achieved by the process features according to claim 1.

Advantageous developments of this method according to the invention are specified in claims 2 to 17 and 39.

• Detektion von Zeitstandschädigungen, in Form von Cavities in metallischen Werkstoffen,
• zur Kontrolle auf Fehler an
  • Elektrobauteilen, wie Chips, Halbleitern, Solarzellen,
  • Lötstellen,
  • Erzeugnissen der Papierindustrie auf Dicke, Faserverteilung, Haftung,
  • Erzeugnissen der Kunststoffindustrie auf Porositäten, Faser- verteilung und Orientierung und
• zur Prozeßkontrolle.

The invention also relates to several advantageous uses of this method according to claims 18 to 22. Thus, the method according to the invention can be used in the context of non-destructive material testing for the detection of material inhomogeneities, material defects, delaminations as well as signs of corrosion and erosion, for example for

• Detection of creep damage, in the form of cavities in metallic materials,
• to check for errors
  • Electrical components such as chips, semiconductors, solar cells,
  • Solder joints,
  • Paper industry products on thickness, fiber distribution, adhesion,
  • Products of the plastics industry on porosity, fiber distribution and orientation and
• for process control.

The method can also be used to determine Material structures, material parameters, such as density, Conductivity, degree of hardness, and to determine material conditions.

Another advantageous use is in the measurement of Layer thicknesses, coverings, surface qualities, for example Surface roughness, and for measuring the adhesion of coatings.

A special use of the method extends to Searching for clues, e.g. for fingerprints. The procedure is according to Another use also suitable for tracking and uncovering counterfeits, e.g. for banknotes, paintings, metal alloys, Coins, ceramics and antique furniture.

The invention further relates to an advantageous device according to claim 23 to carry out the method according to method claims 1 to 17 which explained the subject of claim 1 Underlying task.

Advantageous further developments on the subject matter of claim 23 are given in claims 24 to 37, further preferred Embodiment of a thermal microscope in the sibling Claim 38.

The advantages that can be achieved with the invention are above all in it to see that now laser light sources are relatively lower Power and small size can be used because the External and / or internal measurement head attenuation of the laser beams through the optical fibers and their coupling mechanism and coupling optics are omitted. For example, a diode pumped Neodymium / YAG laser with a wavelength of 1064 nm, which is invisible Emits light in the near infrared range which has a power of only 0.35 W, with a combined Degree of transmission and reflection for the laser beam of 82.9% could be achieved. Inside the measuring head a very precise beam guidance, which is in the sense of a Improves accuracy and sensitivity. For the device for performing the method according to the invention there are a number of advantageous design options. So another in the housing of the measuring head Lasers can be integrated in the form of a pilot laser, which is preferred a diode laser, also of low power, which e.g. in the visible red light range with a wavelength of 670 nm and emits a power of 3 mW. For coupling the pilot beam is a cheap version in which the laser beam, which initially runs axially parallel to the pilot beam, over two Deflecting mirrors connected in series are deflected by 90 ° each is, after the second deflection of the laser beam and Pilot beam are on the same light path, i.e., the second Deflecting mirror is especially for this purpose as a dichroic mirror form which of the laser beam on its reflective side in his further ray path and that on his the other side receives the pilot beam and this like a permeable Windows lets through practically without loss. The pilot beam is very advantageous for adjusting the measuring head, by e.g. a red spot of light on the material sample is thrown and now after this red light spot the scan zone can be chosen. Furthermore, it is convenient after the second Deflecting mirror, i.e. within the common light path for Pilot and laser beam to arrange an optics that together with the optics arranged directly at the laser output are expanded optics forms which advantageously parallel on a short beam path Laser light generated. The expansion optics therefore consists of a Laser output side first optics, which show the beam divergence increased, and the aforementioned, the second deflecting mirror downstream second optics, which parallelizes the laser light. This expansion optic is then optically connected Coupling mirror, from which the laser beam and the pilot beam over the scanner arrangement in the axis of the laser beam end light-guiding optics are thrown. The latter is a lens or a lens system with special properties, which the Laser beam (and of course the pilot beam) in the direction transmits or transmits to the material sample. In the opposite direction, this lens system leaves the IR light signals through. A is suitable for this purpose Zinc selenide glass with a coating on the front. This front coating has the task of IR light transmission to improve in a certain spectral range e.g. in the range 2 - 5 µm. By a different interpretation (lens shape, However, it is also the choice of material and coating) possible to extend this improvement to a larger area especially the range 8 - 12 µm ("second IR window").

The measuring head points in the beam axis of the coupling mirror and of the first scanner arrangement - towards that of the material sample seen arriving IR light signals - a downstream IR deflecting mirror on which the IR light signals e.g. through 90 ° through a deflection mirror connected downstream Forwards the infrared lens to the at least one infrared detector, the infrared lens the IR light signals on the Focused receiving surfaces of the IR detector mentioned. The named IR deflecting mirror can have a normal design, if it only serves to reflect the IR light signals that from the material sample to the scanner arrangement and the coupling mirror be sent to him. Should be via this IR deflecting mirror however, IR radiation can also be injected by a Additional unit in the case of the radiographic test via its internal mirror system is fed, then it is appropriate execute and arrange this IR deflecting mirror so that it in Regarding the second light path as a translucent window acts and in relation to the first IR light path as a mirror.

As mentioned, the laser light source is expediently one Assigned expansion optics for the laser beam. The scanner arrangement preferably has two scanner mirrors, which of associated Drives are moved so that a scanner mirror the beam deflection in the x direction and the other the Beam deflection in the y direction is used.

The measuring head can - as already indicated - by a Additional unit can be added, which - for material samples sufficient wall thickness behind the material sample is positioned and from the back of the material sample emitted IR radiation or corresponding IR light signals receives and has an internal deflection mirror system, which the IR light signals in the IR beam path of the actual Sensor sends or reflects.

FIG 1

eine Einrichtung nach der Erfindung, aufgegliedert in die rechts dargestellte Materialprobe, den in der Mitte dargestellten Meßkopf und die im linken Teil gezeigten transportable elektronische Schrankeinheit, wobei eine Blockschaltbild-Darstellung gewählt ist;

FIG 2

in isometrischer Darstellung einer Computergraphik das Innere des Meßkopfes in detaillierterer Darstellung der strahlführenden Mittel;

FIG 3

die Außenansicht auf eine Einrichtung nach der Erfindung in fotografischer Perspektive, wobei in Abwandlung zu FIG 1 die elektronische Schrankeinheit nicht in Turmbauweise, sondern in Flachbauweise angeordnet ist;

FIG 4

den Gegenstand nach FIG 3 mit einer in Turmbauweise angeordneten Schrankeinheit;

FIG 5

eine Ansicht des portablen Meßkopfes von oben;

FIG 6

eine Darstellung des Meßkopfes bei Betrachtung in Richtung A' von FIG 3 bzw. FIG 4;

FIG 7

einen Schnitt nach der Schnittebene VII-VII aus FIG 5 und

FIG 8

ein Diagramm, welches auf dem Farbmonitor der elektronischen Schrankeinheit dargestellt wurde, wobei die Länge der Abszissenachse des Diagramms 300 µm und die Ordinatenachse 400 µm repräsentiert und wobei Mikroporen verschiedener Größe dargestellt sind, welche an zeitstandsbeanspruchten Rohrleitungen festgestellt wurden;

FIG 9

schematisch in Draufsicht den Meßkopf in einer Abwandlung mit angebauter Stromversorgungseinheit und

FIG 10

eine weitere Abwandlung des Meßkopfes in Draufsicht schematisch, wobei ein Zusatzaggregat zur Ausmessung von Materialproben relativ geringer Wanddicke an den eigentlichen Meßkopf angebaut ist;

FIG 11

perspektivisch-schematisch drei verschiedene Anstrahl- und Abtast-Bahnmuster, die mittels x-/y-Ablenkung verwirklicht werden können.

In the following, a device for carrying out the method according to the invention is first explained using several exemplary embodiments shown in the drawing, then the method itself and further advantages and details are explained. The drawing shows in a partially simplified, schematic representation:

FIG. 1

a device according to the invention, divided into the material sample shown on the right, the measuring head shown in the middle and the portable electronic cabinet unit shown in the left part, a block diagram representation being selected;

FIG 2

in an isometric representation of a computer graphic, the interior of the measuring head in a more detailed representation of the beam guiding means;

FIG 3

the external view of a device according to the invention in a photographic perspective, the electronic cabinet unit being arranged in a flat construction, not in a tower construction, in a modification of FIG. 1;

FIG 4

the object of Figure 3 with a tower unit arranged cabinet unit;

FIG 5

a view of the portable measuring head from above;

FIG 6

a representation of the measuring head when viewed in the direction A 'of FIG 3 or 4;

FIG 7

a section along the section plane VII-VII of Figure 5 and

FIG 8

a diagram, which was shown on the color monitor of the electronic cabinet unit, the length of the axis of abscissas of the diagram representing 300 microns and the ordinate axis of 400 microns and showing micropores of different sizes, which were found on pipeline stressed pipes;

FIG. 9

schematically in plan view of the measuring head in a modification with attached power supply unit and

FIG 10

a further modification of the measuring head in plan view schematically, wherein an additional unit for measuring material samples of relatively small wall thickness is attached to the actual measuring head;

FIG 11

perspective-schematic three different beam and scanning path patterns that can be realized by means of x / y deflection.

FIG. 1 shows in three blocks the essential elements of the method for examining the properties of absorbent materials, in this case material sample B, after the photothermal effect and the device for carrying it out. The measuring head A, highlighted by a black border, is designed as an integral laser measuring head. It contains a directly modulated, diode-pumped neodymium / YAG laser FL, which emits in the 1064 nm wavelength range, ie in the invisible near IR range, and has an output of approx. 0.35 W. This laser light source FL, hereinafter abbreviated as laser, is indicated schematically by a square. A diverging optic 1, the laser beam passes f ₁ to a dielectric mirror 4a which the laser beam f ₁ by 90 ° in the direction of a scanning mirror arrangement 5 deflects. From this, the laser beam passes through a light-guiding optic 6 at the end of the laser beam, shown as a convex lens, which also forms the exit window for the laser radiation f _{1 of} the measuring head A and the entry window for the IR light signals, focused on the front surface bl of the material sample B, namely at a measuring point b2. Above the material sample B, two coordinate axes ± x and ± y are shown in FIG. 11, which are intended to symbolize the deflection system for beam deflection or measuring point scanning in accordance with an irradiation path pattern or a corresponding scanning path pattern for the emitted IR radiation. The beam and scan path patterns preferably run in horizontal or vertical meanders or in spiral paths, as indicated schematically. Other spot and scan path patterns are possible, e.g. B. concentric circles. The two coordinate axes ± x and ± y are dashed and framed by a line 9, which can be a scan zone, for example. Due to the incident laser radiation, which can be modulated according to a certain pulse-pause ratio, and in the respective measuring point a quantity of heat that has an energy of, for example, 2. 10 ^-5 Ws corresponds to, generated, the material sample B emits IR light signals out of phase - Δ f ₃ . The minus sign is intended to symbolize the direction opposite to the incident laser radiation f ₁ . These IR light signals are emitted to the light-guiding optics 6 and let them pass, because this is what is referred to as double optics in the following, which transmits and focuses the laser radiation f ₁ in the direction of the material sample B and which preferably favors in the opposite direction emits IR light signals. This double optics 6 preferably has a coating on its front which acts as a window for a spectral range of 2-5 μm in the infrared range, but practically does not transmit the laser light with its wavelength of 1.064 μm in the beam direction of the IR light signals. For example, this double optic 6 consists of Zn selenide glass and / or Ca fluoride and / or Ba fluoride.

The IR radiation -Δ f ₃ transmitted through the double optics 6 is first thrown onto the scanner mirror arrangement 5 and from there onto a semi-transparent coupling mirror 4a, which is designed as a dichroic mirror which reflects in the direction of the laser beam f ₁ , but in the opposite direction acts as a window that is transparent to the IR light signals. Double optics 6 and coupling mirror 4a act as a decoupling element for the emitted IR light signals. The IR light signals are thrown from the coupling mirror 4a onto an IR deflecting mirror 4b which is optically connected to it and which deflects the IR light signals, for example by 90 °, and directs them to the IR objective 7. By appropriate design of the double optics 6 or by designing an optical system consisting of optics for visible light and for IR light, it is also possible to "run" the IR detection at a time interval that can be adjusted via the local distance from the laser excitation in order to detect a defined depth zone of the sample or to illuminate it - if its wall thickness is not too great - in such a way that it emits or emits IR light signals from its rear. As mentioned, 6 glasses are preferably used for the double optics, the properties of which are designed such that the effect of a collective lens is achieved both for the laser beams f ₁ and for the IR radiation - Δ f ₃ . By changing the aperture of the double optics 6, for example by changing the structure and arrangement of the optical elements, a proportion of the IR light signals emitted by the material sample B that is adapted to the measurement purpose can be enlarged and directed to the IR detector. The function of the double optics 6 can in principle be replaced by holographic / optical elements.

The IR light signals pass through the coupling mirror 4a and the deflection mirror 4b, the latter deflecting the signals mentioned by, for example, 90 °, onto the IR lens 7, which focuses the IR light signals onto the receiving surfaces 8a of the IR detector 8. The IR lens 7 consists, for example, of calcium fluoride, or Ge or Si. The IR detector, which converts the incoming IR light signals into corresponding electrical signals, consists for example of an indium-antimonide compound and has a detection area of approx. 50 - 100 µm in diameter. Its signal / noise ratio is most favorable at a working temperature of approx. 100 K. This temperature is approximately achieved by cooling with nitrogen. A corresponding detector cooling unit is indicated at 10; the corresponding cooling gas supply and discharge line is designated 11. The IR detector is cooled in particular with a Joule-Thomson cooling (using N ₂ ). Instead, for example, a Stirling cooler based on the Stirling engine principle can also be used.

The electrical signals generated in the IR detector pass through a signal line 12 to one arranged inside the measuring head A. Preamplifier 13 small size, and from the output of Preamplifiers 13 are the pre-amplified electrical ones IR light signals analog signals via the signal line 14 an electronic amplifier stage, in particular a lock-in amplifier 15 forwarded, the latter within one portable electronic cabinet unit C is housed.

When using the diode-pumped solid-state laser FL or when using a laser diode that is also suitable for the intended purpose, the modulation of the laser beam f _{1 can be achieved} via the electrical circuit of the laser; a separate modulator or chopper is then not required.

The housed inside the electronic cabinet unit C. Amplifier stage 15 is e.g. a digital lock-in amplifier (DLI). In contrast to thermography (in which the temporal largely constant temperature is detected) photothermal measuring method the amplitude and phase of the Temperature modulation determined. The phase shift results derive from the time delay with which the maximum Surface temperature compared to the time of excitation is measured. The phase shift is done with the lock-in amplifier certainly. In electrical connection in both This amplifier 15 has signal directions with a module 16 "device control", and this control module 16 is again electrically and electronically interconnected with an integral electrical signal processing and storage unit 17 with Screen or monitor 18. The unit 17 is in particular a Personal computer (PC). In other words: the transportable electronic cabinet unit C includes means 15, 17 for electronic Signal processing, storage and display of electrical at least one IR light detector 8 supplied Signals and second means 16 for controlling the measuring head A. These include: At least one electronic amplifier stage 15 and an associated electronic computing unit 17, further that between the amplifiers 13, 15 and the electronic Computer unit 17 turned on control module 16. On the Screen 18 shows the IR light signals collected and processed data presented. The control module 16 generates the control signals for setting the Laser beam characteristics for the Laser FL, such as pulse-pause ratio and beam power, beam path pattern, and scan path pattern as well as scan speed.

By eliminating the separate gas laser and the No transmission of laser radiation via optical fibers As already mentioned, device mobility can be established.

The electronic cabinet unit C and the integral measuring head A only by a highly flexible electrical cable C1 (cf. FIGS. 3 and 4) a relatively large range with one another connected. This also means that the electronic cabinet unit C possible, measuring points in wide range can be reached.

In Figure 1 with 19 are an electrical signal line between the amplifier 15 and the laser FL and designated 20 one further electrical signal line for controlling the mirror scanner arrangement 5 of corresponding signal output terminals of the personal computer or the computing unit 17. In figure 1 is a cable for supplying the measuring head A with electrical Energy not shown separately. Such a power supply cable but is in the flexible connection cable C1 Figure 3 and Figure 4 included.

In the exemplary embodiment according to FIG. 2, which is more detailed in comparison to FIG. 1, two optically connected deflection mirrors 2a, 2b and an expansion lens 3 downstream of the second deflection mirror 2b can be seen. Furthermore, the mirror scanner arrangement 5, which consists of two scanners, can be seen more clearly -Mirrors 5a, 5b, which are optically connected in series, the scanner mirror 5a is mounted adjustably about the axis of rotation 21 and the scanner mirror 5b is rotatably mounted about the axis of rotation 22, so that the scanner mirror 5a on it thrown laser beam f ₁ in the direction ± x and the scanner mirror 5b adjusts the laser beam thrown onto it in the direction ± y (cf. FIG. 11). In Figure 2, a housing 23 for the laser FL is indicated in its outline and a pilot laser DL structurally combined with this housing 23 in the form of a diode laser, from which a pilot beam f ₂ takes its exit, which enters the beam path via the second deflecting mirror 2b of the laser beam f _{1 is} injected. As can be seen, the beams f ₁ (laser beam) and f ₂ (pilot beam) initially run parallel to one another. The laser beam f _{1 is} deflected twice by 90 ° by the two successively connected first and second deflection mirrors 2a, 2b, and after the second deflection (after the second deflection mirror 2b), both beams f ₁ and f ₂ collide with one another. The second deflection mirror 2b is designed to couple in the pilot beam f ₂ as a dichroic mirror which reflects the laser beam f ₁ with its reflection side, but at the same time allows the pilot beam f ₂ arriving from the other side to pass through as a transparent window. Both beams f ₁ , f ₂ then pass through the expansion optics 3 and reach the dielectric mirror 4a. In practice, the pilot laser DL is only switched on when the scan zone 9 (FIG. 1) is to be defined. The pilot beam f ₂ thus arrives (when the laser FL is not yet working and the pilot laser DL is switched on) via the dielectric coupling mirror 4a and the scanner mirror arrangement 5 through the double optics 6 on the material sample and thereby generates a red dot, for example. When the scan zone is defined, the pilot laser DL should preferably continue to be operated so that the actual measurement process can be followed when it starts after the laser FL is switched on. In FIG. 2, a number of desk-type bearing elements for the above-described optics are indicated in a kind of phantom representation and are generally designated by 24.

In FIG. 2, the laser beam f ₁ and the pilot beam f ₂ and the beam of the IR light signals emitted by the material sample (not shown in FIG. 2) are highlighted by means of reinforced lines. The beam paths of all three beam types are common between the coupling mirror 4a and the double optics 6; only IR radiation exists in front of the coupling mirror 4a up to the IR detector 8. For the table below it is assumed that (+) the beam direction is in the direction of the material sample and (-) the direction of the IR light signals emitted by the material sample. The following table provides a clear summary of the properties of the individual optics as well as the optical conditions for the three beam types f ₁ , f ₂ and - Δf ₃ and also shows in the right column the degrees of transmission of the optics and the reflectance of the mirrors for the laser radiation f ₁ on. Multiplying the values in the right column gives a resulting combined (first) transmittance and reflection factor for the laser beam f ₁ of 0.829 and 82.9%. For the IR beam - Δf _{3 there} is a similarly favorable resulting (second) value because the transmission and reflection levels of the optics 6, 7 and the mirrors 5b, 5a, 4a, 4b for the IR beam are comparable to the transmissions and degrees of reflection of the corresponding optics and mirrors for the laser beam f ₁ . Opt. Element translucent in reflected in Transmission or reflex. degree ff ₁ Direction (+) Direction (-) Direction (+) (-) Expanding optics 1 f ₁ - 0.99 Deflecting mirror 2a - f ₁ 0.998 Deflecting mirror 2b f ₂ f ₁ 0.998 Expanding optics 3 f ₁ , f ₂ - 0.99 dichroic mirror 4a ("beam splitter") - -Δ f ₃ f ₁ , f ₂ 0.96 Scan mirror 5a - - f ₁ , f ₂ -Δ f ₃ 0.96 Scan mirror 5b - - f ₁ , f ₂ -Δ f ₃ 0.96 Double optics 6 f ₁ , f ₂ -Δ f ₃ - - 0.96 IR deflecting mirror 4b - - - -Δ f ₃ IR lens 7 - -Δ f ₃ - - f ₁ = frequency of the laser f ₂ = frequency of the pilot laser -Δ f ₃ = IR radiation

In Figures 3 and 4 are the same parts to Figure 1 with the provided with the same reference numerals. In both representations is the portable measuring head A with cooling slots 25 for removing the Heat loss provided; he is on platform 26 a tripod 27 and arranged with a flexible cable C1 the electronic cabinet unit C connected.

From Figures 5 to 7 you can see further details, in particular of a constructive nature, the measuring head A. Figure 5 shows the double optics 6 on the front Al of the measuring head A, the Coupling mirror 4a and the IR deflection mirror 4b and the scanner mirror arrangement 5. In Figure 6, the drive 5bl for one scanner mirror 5b and the other scanner mirror 5a to see, as well as the double optics 6. Figure 7 shows in their Outlined the laser FL with its expansion optics 1, the two him downstream deflection mirror 2a, 2b, the coupling mirror 4a, the IR deflecting mirror 4b (which is circular in the drawing is) and the second optics 3, which are part of the expanding optics is.

FIG. 8 shows the snapshot of a scan zone, enlarged, namely, micro-pores found in creep stresses Pipelines.

FIG. 9 shows that an energy supply unit is attached to the measuring head A. AO can be attached so that no power supply cable to the measuring head must be relocated. This supply unit AO can contain rechargeable batteries.

Figure 10 shows that there is a separate, at the actual Measuring head attachable unit A2 is possible, relatively thin-walled Material samples B 'on the radiated from their back To investigate IR radiation, which via the IR optics 26, the two deflecting mirrors 27, 28 and a further IR optic 29 via an entry window 30 of the measuring head A onto the deflecting mirror 4b from the back. This deflecting mirror 4b is a dichroic mirror for this application, where then the further beam path to the IR lens 7 and IR detector 8 is as explained with reference to Figure 2.

With the device according to FIG. 10, a method is implemented in which the material sample B 'is illuminated on a front side with the laser beam f _{1 in} accordance with an illumination path pattern. With a correspondingly small material wall thickness of the material sample B '(e.g. fractions of a millimeter), the IR light signals - Δ f ₄ emitted from the back of the material sample B' can now be scanned in accordance with a scanning path pattern. This can e.g. B. happen so that the deflecting mirrors 27, 28 are designed as scanner mirrors, which are deflected according to the scanner mirrors 5a, 5b according to Figure 2 synchronously with them in the x or y direction by small amounts by motor. As can also be seen from FIG. 10, it is advantageous if the IR light signals - Δ f ₄ emitted from the back of the thin-walled material sample B 'are deflected such that they emitted with the last part of the beam path for the material sample side facing the laser IR light signals - Δ f ₃ , which is aligned with the IR detector 8, coincide. In the present case, the union takes place at the IR deflecting mirror 4b. By means of light barriers not shown in FIG. 10 it can be achieved that either only the IR light signals - Δ f ₃ or - Δ f _{4 reach} the IR detector 8 via the IR deflecting mirror. One can therefore examine the material sample B 'according to both process variants: either the IR light signals - Δ f ₃ , which are emitted from the laser-facing side of the material sample B', or the IR light signals - Δ f ₄ , which are emitted side of the material sample B 'facing away from the laser. A space 31 is provided between the measuring head A and the additional unit for inserting the thin-walled material sample B '. The IR light signal - Δ f ₄ received by the additional unit A2 via the IR optics 26 is deflected by two 90 ° through a first housing-sealing light-conducting optics 29 from the additional unit into the intermediate space 31 and from there via a second housing-sealing optics 30 into the internal beam path of the IR light signals - Δ f _{3 of} the measuring head A. This has the advantage that a single IR detector 8 is sufficient. In special cases, however, a separate IR detector can be assigned to the additional unit A2, so that a material sample B 'can be examined practically simultaneously from both sides. The additional unit A2 can also be designed so that it can be swung in and out with respect to the axis of the laser beam f ₁ , so that the measuring head A is suitable either for surface examinations or for radiographic examinations of thin-walled material samples.

Returning to the preferred exemplary embodiment of the "thermal microscope" according to FIGS. 1 to 7, the following explains again why the omission of optical fibers in the beam path of the laser beam f ₁ within the portable measuring head and the preferred omission of these optical fibers in the beam path emitted by the material sample IR light signals - Δf ₃ up to the IR light detector 8 is particularly advantageous. Optical fibers for the laser light must be so-called monomode fibers. They act like a waveguide, which means that the rays pass through the core practically in a straight line. The coherence properties of the laser light are not or only slightly impaired. This requires a core diameter of a few µm, i.e. a few thousandths of a millimeter. Such a fiber is required to enable diffraction-limited focusing and, consequently, high lateral resolution. The disadvantages of single-mode fibers are a complex coupling mechanism and optics between the laser and the fiber entrance, as well as the high coupling losses that still arise. As measured in the laboratory under optimal conditions, they are 30 to 40%, in practical use, however, they are 50%. Compared to this, the attenuation losses in the fiber are relatively low (30 dB / km at 488 nm, 2 dB / km at 1064 nm).

The thermal microscope according to the present invention a collinear arrangement (match of laser and IR path between dichroic mirror 4a and double optics or scan lens 6) before, which with fiber optic technology is not can be realized since an IR optical fiber is not simultaneously Single mode fiber for the laser wavelength can be. You could at best remember between dichroic mirror 4a and IR detector 8 to install an IR optical fiber. Thereby the IR detector could be outside of the portable measuring head be placed. However, the others would be disadvantageous Transmission losses and the low mechanical resilience of IR fibers. There would also be a connecting line between the electronic components including IR detector and to make the portable measuring head hardly pluggable, but should be installed permanently.

a) das Wärmemikroskop weist einen Laser FL zur Aussendung eines modenreinen Laserstrahls f₁ auf, ferner

b) eine mechanische oder optische Scanneranordnung 5; 5a, 5b zur Ablenkung des Laserstrahl f₁.

c) Eine Optik 6 ist vorgesehen,

c1) zur Fokussierung des Laserstrahls f₁ auf einer Materialprobe B in einem Meßpunkt b2 mit einem Fokusdurchmesser, der kleiner/gleich 10µ beträgt und

c2) zur Zurückleitung der von der Materialprobe B emittierten Infrarot-Lichtsignale - Δ f₃.

d) Ein Auskoppelelement 4a zur Auskopplung der Infrarot-Lichtsignale - Δ f₃ ist ferner beim Wärmemikroskop vorgesehen ebenso wie

e) ein Infrarot-Detektor 8, der neben dem Laser FL angeordnet ist. Außerdem weist das Wärmemikroskop auf:

f) einen Umlenkspiegel 4b zur Umlenkung der vom Auskoppelelement 4a ausgekoppelten Infrarot-Lichtsignale - Δ f₃ auf den Infrarot-Detektor 8,

g) ein Gehäuse, in dem der Laser FL, die Scanner-Anordnung 5; 5a, 5b, die Optik 6, das Auskoppelelement 4a, der Umlenkspiegel 4b und der Infrarot-Detektor 8 gemeinsam untergebracht sind, und schließlich

h) eine Signalauswerte-Einheit 13, 15, 17, 18 zur Auswertung und Darstellung der Signale des Infrarot-Detektors 8.

In summary, the preferred embodiment of the device according to the invention, which can be referred to as a "thermal microscope", can be characterized by the following features:

a) the thermal microscope has a laser FL for emitting a single-mode laser beam f ₁ , furthermore

b) a mechanical or optical scanner arrangement 5; 5a, 5b for deflecting the laser beam f ₁ .

c) an optical system 6 is provided,

c1) for focusing the laser beam f ₁ on a material sample B in a measuring point b2 with a focus diameter which is less than / equal to 10μ and

c2) for returning the infrared light signals emitted by the material sample B - Δ f ₃ .

d) A decoupling element 4a for decoupling the infrared light signals - Δ f ₃ is also provided in the thermal microscope as well

e) an infrared detector 8 which is arranged next to the laser FL. The thermal microscope also features:

f) a deflecting mirror 4b for deflecting the infrared light signals - Δ f ₃ decoupled from the decoupling element 4a - onto the infrared detector 8,

g) a housing in which the laser FL, the scanner assembly 5; 5a, 5b, the optics 6, the decoupling element 4a, the deflecting mirror 4b and the infrared detector 8 are housed together, and finally

h) a signal evaluation unit 13, 15, 17, 18 for evaluating and displaying the signals of the infrared detector 8.

As can be seen in process claims 1 and 39 as well as in the accompanying text to the table on page 17 and from the right column of this table, very good resulting transmission and reflection levels of at least 60% can be achieved with the thermal microscope according to the invention, both for the light path of the laser beam f ₁ and the light path of the emitted IR light signals -Δ f ₃ . The first resulting degree of transmission and reflection for the laser beam f ₁ can, even with good quality of the optics and coatings used, be in a range between 60 and 85% and is preferably at least 80%. The corresponding values for the IR beam are somewhat lower, but are quite comparable with the favorable transmission and reflection levels that can be achieved for the laser beam f ₁ .

Claims (39)

1. Method for testing the properties of materials according to the photothermal effect, having the following method features:

   a) emission of a laser beam (f₁) in the direction of the surface region (9) to be tested of the material sample (B), with the laser beam (f₁) being focused to the desired measuring point diameter in the target light spot by means of a photoconductive optical element (6) on the laser-beam-end side, so that there a portion of the amount of light energy is absorbed by the irradiated volume elements of the material sample, and infrared (IR) light signals (-Δf₃) are emitted by the surface of the latter and the volume elements adjacent thereto;

   b) simultaneous generation of an irradiation path pattern of the laser radiation (f₁) and of a radiation path pattern of the emitted IR light signals (-Δf₃) by a scanner arrangement (5), which is arranged inside a portable measuring head (A) and has at least one mirror, which is rotatable about an axis and through which the laser radiation (f₁) and the IR light signals ( -Δf₃ ) are conducted;

   c) conduction of the IR light signals (-Δf₃), which have been emitted and conducted through the scanner arrangement (5), to an optical decoupling element (6, 4a), by which the emitted IR light signals (-Δf₃) are conducted further and portions of the laser beam (f₁) that are reflected on the surface of the sample are substantially suppressed;

   d) further conduction of the decoupled IR light signals (-Δf₃) and focusing thereof onto the receiving surfaces (8a) of at least one IR light detector (8), which converts the received IR light signals into corresponding electrical signals for the purpose of the further signal conditioning; and

   e) integration of a laser light source (FL) into the housing of the portable measuring head and conduction of the laser beam (f₁) from the laser light source (FL) up to the photoconductive optical element (6), on the laser-beam-end side, inside the portable measuring head (A) with a first resulting transmittance and reflectance of at least 60%, and conduction of the IR light signals (-Δf₃), which are emitted by the material sample (B), to an IR light detector (8) inside the portable measuring head (A) with a second resulting transmittance and reflectance of at least 60%.
2. Method according to claim 1, characterised in that a diode-pumped solid-state laser is used as a laser light source (FL).
3. Method according to claim 1, characterised in that a diode laser is used as a laser light source (FL).
4. Method according to one of the claims 1 to 3, characterised in that a face of the material sample (B') is irradiated with the laser beam (f₁) according to the irradiation path pattern, and in that the IR light signals (-Δf₄) which - in the case of a correspondingly small material wall thickness - are emitted by the rear side of the material sample (B') are scanned according to the scanning path pattern.
5. Method according to one of the claims 1 to 3, characterised in that the material sample (B) is irradiated on the same side by the laser beam (f₁) and scanned with regard to the IR light signals (-Δf₃) emitted by it, and in that the emitted IR light signals, on part of their way, are conducted along the same beam path and through at least one photoconductive optical element on the laser-beam-end side, as used for conducting the incoming laser radiation (f₁) in accordance with the irradiation path pattern, to the optical decoupling element.
6. Method according to one of the claims 1 to 3 and 5, characterised in that the optical decoupling element (4a, 6) comprises a so-called doublet (6) which lets through and focuses the laser radiation (f₁) in the direction of the material sample (B), and in the opposite direction preferentially lets through the emitted IR light signals (-Δf₃).
7. Method according to one of the claims 1 to 6, characterised in that in order to generate the irradiation path pattern of the laser radiation (f₁), the latter is conducted via two scanner mirrors (5a, 5b) which are optically connected in series, one (5a) of which mirrors is rotated about a first axis (21) for beam deflection in the X direction, and the other (5b) is rotated about a second axis (22) for beam deflection in the Y direction.
8. Method according to one of the claims 1 to 3 and 5 to 7, characterised in that the optical decoupling element (6, 4a) comprises a dichroic coupling mirror (4a), and the laser radiation (f₁) in a beam direction pointing towards the material sample (B) is reflected by the active mirror surface of the coupling mirror (4a) into the beam path which is aligned with the scanner mirrors (5a, 5b), while on the other hand, the IR light signals (-Δf₃) arriving in the opposite direction to the laser beam (f₁) are let through by the coupling mirror (4a), which acts as a window in this direction.
9. Method according to one of the claims 1 to 8, characterised in that the laser beam (f₁) exiting from the laser light source (FL) passes through a first optical element (1), which expands the laser beam and is part of an expanding optical element (1, 3), and is subsequently deflected twice, by 90° in each case, by way of two deflecting mirrors (2a, 2b), which are optically connected in series, so that it is aligned with the active surface of the coupling mirror (4a), and from there makes its way via the two scanner mirrors (5a, 5b) to the photoconductive optical element (6) on the laser-beam-end side, by which it is focused onto the measuring points on the material sample (B).
10. Method according to claim 9, characterised in that the laser beam (f₁) is guided from the second (2b) of the deflecting mirrors (2a, 2b) to the coupling mirror (4a) by way of a second optical element (3), which is part of the expanding optical element (1, 3).
11. Method according to claim 9 or 10, characterised in that the pilot beam (f₂) of a pilot laser (DL) is coupled into the beam path of the laser beam (f₁), so that before the start of the testing or irradiation of the material sample (B) with the laser beam (f₁), there is a visible target pilot light spot on the material sample for the adjustment of the scanning zone (9).
12. Method according to claim 11, characterised by the use of a pilot laser (DL), preferably a diode laser, which emits in the visible red range.
13. Method according to claim 11 or 12, characterised in that the pilot beam (f₂) is irradiated into the beam path between the second deflecting mirror (2b) and the coupling mirror (4a), and in that for this purpose, the second deflecting mirror (2b) lets through, in the manner of a window, the pilot beam (f₂) arriving on its rear side, while it reflects onto the coupling mirror (4a) the laser beam (f₁) arriving on its front side.
14. Method according to one of the claims 1 to 13, characterised in that the laser beam (f₁) is modulated in order to achieve a desired mark-to-space ratio.
15. Method according to one of the claims 1 to 14, characterised in that the laser beam (f₁) is guided over the scanning zone (9) along an orthogonal irradiation path pattern.
16. Method according to one of the claims 1 to 14, characterised in that the laser beam (f₁) is guided over the scanning zone (9) along an irradiation path pattern which is formed by spiral or concentric circular paths.
17. Method according to claim 4, characterised in that the IR light signals (-Δf₄) emitted by the rear side of the thin-walled material sample (B') are deflected in such a way that they coincide with the last portion of the beam path for the IR light signals (-Δf₃) emitted by the side of the material sample that faces the laser, which beam path is aligned with the IR detector (8).
18. Use of the method according to one of the claims 1 to 17 within the scope of non-destructive material testing for detecting material non-homogeneities, flaws in the material, delaminations as well as corrosion marks and erosion marks, for example for:

    detecting creep damages in the form of cavities in metallic materials

    checking for flaws in

    electrical components such as chips, semiconductors, solar cells

    soldered joints

    products of the paper industry with regard to thickness, fibre distribution, adhesion,

    products of the plastics industry with regard to porosities, fibre distribution and orientation, and

    for process control.
19. Use of the method according to one of the claims 1 to 17 for establishing material structures, material characteristics such as density, thermal conductivity, specific heat, degree of hardness, for example, and for establishing material conditions.
20. Use of the method according to one of the claims 1 to 17 for measuring layer thicknesses, coatings, surface qualities, for example peak-to-valley heights, and for measuring the adhesion of coatings.
21. Use of the method according to one of the claims 1 to 17 for searching for traces, for example for fingerprints.
22. Use of the method according to one of the claims 1 to 17 for tracing and uncovering forgeries, for example in bank notes, paintings, metal alloys, coins, ceramics and antique furniture.
23. Device for carrying out the method according to one of the claims 1 to 16, having a portable measuring head which has a compact housing, with the housing having an end wall, comprising:

    a) a laser light source, accommodated in the housing of the portable measuring head, the laser beam (f₁) of which laser light source is conducted without optical fibres by way of beam-guiding means (1, 2a, 2b, 3, 4a, 5a, 5b), which are inside the housing, up to a photoconductive optical element (6) on the laser-beam-end side;

    b) the above-mentioned photoconductive optical element (6) on the laser-beam-end side, accommodated on or in the end wall (A1) of the housing and constructed as a so-called doublet, which lets through and focuses the laser radiation (f₁) in the direction of the material sample (B), and in the opposite direction preferentially lets through the IR light signals (-Δf₃) emitted by the material sample, with the optical element (6) being part of an optical decoupling element (6, 4a);

    c) at least one IR light detector, accommodated in the housing of the measuring head and having its receiving surfaces aligned with the focus point of an infrared objective connected optically upstream for converting the IR light signals into corresponding electrical signals for the purpose of further signal conditioning, with the beam-guiding means inside the housing comprising a scanner mirror arrangement (5), which is connected upstream of the optical element, which is on the laser-beam-end side, in the beam path of the laser beam (f₁) and is arranged for deflecting the laser beam (f₁) in accordance with an irradiation path pattern and for scanning the IR light signals (-Δf₃), which are emitted by the material sample (B) in the region of its scanning zone (9), according to a scanning path pattern; and with the means for conducting the laser beam (f₁) being provided with a first resulting transmittance and reflectance of at least 60% and the means for conducting the IR light signals (-Δf₃) being provided with a second resulting transmittance and reflectance of at least 60%.
24. Device according to claim 23, characterised in that the beam-guiding means inside the housing additionally comprise:

    a1) a coupling mirror (4a), constructed as a dichroic mirror, which is connected upstream of the scanner mirror arrangement (5) in the beam path of the laser beam (f₁) and reflects in the direction of the laser beam (f₁) and in the direction opposite thereto acts as a window which is transparent with regard to the IR light signals;

    a2) an infrared objective (7) which, in a direction opposite to the direction of the laser beam (f₁), i.e. on the side of the coupling mirror (4a) that faces away from the reflecting surface, is connected upstream of said coupling mirror and focuses the IR light signals arriving from the coupling mirror (4a) onto the receiving surfaces (8a) of at least one IR light detector (8).
25. Device according to claim 23 or 24, characterised by at least one transportable electronic cabinet unit (C), comprising first means (15, 17) for electrical signal processing of the electrical signals delivered by the at least one IR light detector (8) and second means for controlling the measuring head, with the first means comprising: at least one electronic amplifier stage (15) and one electronic computer unit (17), and with the second means comprising: a control module (16) connected between amplifier stage (15) and electronic computer unit (17), with the electronic computer unit (17) having at least one screen (18) on which can be displayed the data which has been obtained from the IR light signals, collected and conditioned, and with the control module (16) generating control signals for adjusting the laser-beam characteristics of the laser light source (FL), such as mark-to-space ratio and radiant power, irradiation path pattern and scanning path pattern as well as scanning rate; at least one flexible, electrical interconnecting cable for signal transport between the measuring head (A) and the electronic cabinet unit (C), and means for supplying the portable measuring head (A) with electrical energy from an energy supply source.
26. Device according to claim 25, characterised in that accommodated in the portable measuring head (A) is a pre-amplifier which is electrically connected downstream of the IR light detector and the output signal line (14) of which is led to the amplifier stage (15) of the electronic cabinet unit (C).
27. Device according to claim 25 or 26, characterised in that the amplifier stage (15) is a lock-in amplifier.
28. Device according to one of the claims 23 to 27, characterised in that accommodated inside the portable measuring head (A) is a pilot laser (DL), the pilot beam (f₂) of which can be coupled into the beam path of the laser beam (f₁), and which is used for adjustment of the scanning zone (9).
29. Device according to claim 28, characterised in that the pilot laser (DL) radiates in the visible red range.
30. Device according to claim 28 or 29, characterised in that the pilot beam (f₂) of the pilot laser (DL) is aligned with the beam path for the laser beam (f₁) of the laser light source (FL) that is situated between a second deflecting mirror (2b) and the coupling mirror (4a), with the second deflecting mirror (2b) forming a transmission window for the pilot beam (f₂), which arrives on its rear side, and casting the latter onto the coupling mirror (4a).
31. Device according to one of the claims 23 to 30, characterised in that a cooling unit (10) is provided in order to maintain a low-temperature working range for the IR light detector.
32. Device according to one of the claims 28 to 30, characterised in that in the beam path of the laser (f₁) onto a first optical element (1), which is part of an expanding optical element, there are arranged one behind the other at the output of the laser light source (FL) two first and second deflecting mirrors (2a, 2b), which are optically connected in series and direct the laser beam (f₁) into an offset path which is aligned with the coupling mirror (4a), with the pilot beam (f₂) of the pilot laser (DL) being aligned so as to radiate into this offset path.
33. Device according to claim 32, characterised in that between the second deflecting mirror (2b) and the coupling mirror (4a), a second optical element (3), which is part of the expanding optical element system (1, 3), is inserted into the beam path.
34. Device according to one of the claims 23 to 33, characterised in that in the beam path of the IR light signal (-Δf₃), there is connected downstream of the coupling mirror (4a) an IR deflecting mirror (4b), which receives the IR light signals let through by the coupling mirror (4a) and casts them in the direction of the IR light detector (8) or onto an infrared objective (7) connected upstream thereof.
35. Device according to one of the claims 23 to 34, characterised by a supplementary unit (A2) for the measuring head (A) for photothermal measuring of relatively thin-walled material samples (B') by receiving the IR light signals (-Δf₄) emitted by the rear side of the material sample (B'), comprising an infrared optical element (26) for receiving the IR light signals, a deflecting mirror arrangement (27, 28) for deflecting the IR light signals into a beam axis which is in alignment with the receiving surface (8a) of the IR light detector (8).
36. Device according to claim 35, characterised in that the IR deflecting mirror (4b) arranged upstream of the infrared objective (7) is reflective with regard to the IR light signals (-Δf₃) emitted by the side of the material sample that faces the laser and transparent with regard to the IR light signals (-Δf₄) emitted by the side of the material sample that faces away from the laser.
37. Device according to claim 35 or 36, characterised in that there is provided between measuring head (A) and supplementary unit (A2) a gap for inserting the thin-walled material sample (B'), and in that the IR light signal (-Δf₄) received by the supplementary unit (A2) can be conducted through a first photoconductive optical element (29), which seals off the housing, from the supplementary unit (A2) into the gap, and from here via a second photoconductive optical element (30), which seals off the housing, into the internal beam path of the IR light signals (-Δf₃) of the measuring head (A).
38. Heat microscope

    a) having a laser (FL) for emitting a pure-mode laser beam (f₁),

    b) having an optical element (6)

    b1) for focusing the laser beam (f₁) onto a material sample (B) at a measuring point (b2) having a focus diameter which is less than or equal to 10 µ, and

    b2) for returning the infrared light signals (-Δf₃) emitted by the material sample (B),

    c) having a scanner arrangement (5; 5a, 5b) having at least one mirror which is rotatable about an axis, for deflecting the laser beam (f₁) and the emitted IR light signal (-Δf₃),

    d) having a decoupling element (4a) for decoupling the infrared light signals (-Δf₃),

    e) having an infrared detector (8), which is arranged next to the laser (FL),

    f) having a deflecting mirror (4b) for deflecting onto the infrared detector (8) the infrared light signals (-Δf₃) which have been decoupled by the decoupling element (4a),

    g) having a housing in which the laser (FL), the scanner arrangement (5; 5a, 5b), the optical element (6), the decoupling element (4a), the deflecting mirror (4b) and the infrared detector (8) are jointly accommodated, and

    h) having a signal evaluating unit (13, 15, 17, 18) for evaluating and displaying the signals of the infrared detector (8).
39. Method according to claim 1, characterised in that the first resulting transmittance and reflectance for the laser beam (f₁) lies in the range between 60% and 85%, and preferably amounts to at least 80%.

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